The grease composition is widely used to lubricate various machines such as cars, electrical equipment, construction machines, machine tools, and the like. Machines are becoming more compact, more lightweight, and have increasingly high performance year by year. Of these machines, an electric motor and the like have become compact and are rotated at a high rotational speed and in addition composed of a large number of parts intensively disposed, which leads to the rise of the atmospheric temperature. Therefore the lubricating conditions of grease used for bearings of the electric motor and the like are increasingly severe. In consideration of the above-described situation, improvement of the life of the grease at high temperatures greatly contributes to the improvement of the quality and reliability of machines and is thus very important.
There are some methods of lengthening the life of the grease. A first method is to improve thermal stability by effectively adding a proper antioxidant to the grease. For example, grease using an alkylthio-1,3,5-triazine compound excellent in thermal stability was proposed (see patent document 1). A second method is to obtain grease having a long life by selecting the thickener excellent in heat resistance. For example, there is proposed grease excellent in its thermal stability developed owing to the finding of a urea compound excellent in its heat resistance (see patent documents 2 and 3).
A third method is to use synthetic lubricants excellent in the thermal stability thereof as the base oil of grease or obtain grease having a long life by combining them with each other. It is well known that by using apart or all of synthetic lubricants (for example, poly-α-olefin, diphenyl ether, diester, polyol ester, silicone, fluorinated oil) known for its favorable thermal stability as the base oil of the grease, the heat resistance of the grease is improved and its life can be lengthened.
In a very cold region, to start a car at a very low temperature is considered. When a pulley and the like of equipment driven by an engine of the car is operated in this condition, there is a case where in dependence on a specification and an operation condition, an abnormal sound (whistling), a so-called cold-time abnormal sound is generated. Although the cause of the generation of the cold-time abnormal sound has not yet been clearly elucidated, it is conceivable that the pulley and the like resonate owing to self-excited vibration of a rolling element caused by uneven oil film of grease, and an outer ring axially oscillates (translational motion) to generate the cold-time abnormal sound.
In recent years, cars have high performance, and high-power cars have increased. Thus a load is increasingly applied to a constant-velocity joint (constant-velocity universal joint), and the lubricating condition thereof tends to become more severe. There is a tendency that cars are demanded to give a good ride to a higher extent. It is necessary to make a driver feel comfortable than ever in regions of all climates ranging from a very hot region to a very cold region. When the constant-velocity joint rotates in such a condition, there is a case in which a rotation resistance fluctuates owing to a difference among parts composing the constant-velocity joint. When the rotation resistance fluctuates, there is a case in which a stick slip sound is generated, which is a factor causing the driver to feel less comfortable. Grease containing mineral oil as its base oil and a molybdenum compound as its additive was heretofore disclosed (for example, see patent documents 4 and 5).
Trailers and freight trains transport cargos over a long distance. These vehicles travel between remote locations from a very hot area to a very cold area at high speeds for a long time with the vehicles being subjected to a wide temperature change accompanied by a climate change. In these vehicles, fretting is liable to occur on a rolling surface of a bearing owing to micro vibrations which occur during transport. In very cold regions, because grease hardens, lubricating oil cannot be sufficiently supplied to the rolling surface. Thus the rolling surface suffers greatly from the fretting.
Because a main shaft-supporting bearing wind power generation-is-operated in environment where the di-reaction of a wind and the velocity thereof constantly fluctuate, a load to be applied to the bearing fluctuates owing to fluctuations in the velocity thereof. As a result, a flange wears owing to a reciprocating sliding motion, and the fretting tends to occur on the rolling surface of the bearing. In wind power generators installed on mountains and in very cold regions and subjected to a very low temperature, grease hardens, and the lubricating oil cannot be sufficiently supplied to the rolling surface. Thus the rolling surface suffers greatly from the fretting.
Various methods have been proposed to prevent the occurrence of the fretting. In one of these proposed methods, a method of preventing the fretting by selecting an appropriate lubricant has been proposed. According to the description of the above-described fretting prevention method, the grease in which at least one substance selected from among oxidized paraffin, diphenylhydrogen phosphite, and hexamethylphosphoric triamide is added to the urea-based thickener has fretting prevention property (see patent document 6).
In a hub bearing for a car for rotatably supporting wheels thereof, carbon steel for machine structural use such as S53C which has a good forgeability and is inexpensive is used in a second generation hub bearing (-GEN 2) and a third-generation hub bearing (GEN 3) provided with a flange on the outer ring thereof. In the carbon steel for machine structural use, by subjecting the raceway to high frequency heat treatment, the rolling fatigue strength of the bearing portion is secured. Because the carbon steel for machine structural use contains a small amount of an alloy component, it has a weak surface strength and is inferior to bearing steel in its resistance to peeling of grease arising from the surface of the bearing. As a measure for solving the above-described problem of the carbon steel for machine structural use, grease (see patent document 7) containing zinc dithiocarbamate added to its base oil as its essential component is known.